Method for reducing heterostructure acoustic charge transport device SAW drive power requirements

ABSTRACT

A HACT device employing a thin-film overlay of a more strongly piezoelectric material can operate as a delay line and as a tapped delay line, or transversal filter, while requiring less total power for the SAW clock signal. The increased electrical potential per unit total SAW power thus realized facilitates coupling between the total SAW energy and the mobile charge carriers. Some materials systems, such as a GaAs substrate and a ZnO thin-film overlay, will require an intervening thin-film dielectric layer in between the HACT substrate and epitaxial layers and the thin-film piezoelectric overlay. This may be necessitated by chemical, semiconductor device processing, or adhesion incompatibilities between the substrate material and the thin-film overlay material.

This is a division of application Ser. No. 577,180, filed Sep. 4, 1990.

BACKGROUND OF THE INVENTION

The present invention pertains to a heterostructure acoustic chargetransport (HACT) devices and more particularly to an arrangement forimproving device performance by reducing the magnitude of the surfaceacoustic wave (SAW) required for effective HACT device operation.

An HACT device employs a powerful ultra high frequency (UHF) SAWpropagating on the top, highly polished surface of a wafer ofpiezoelectric semiconductor material, usually gallium arsenide (GaAs),to bunch mobile charge carriers in the extrema of the SAW electricalpotential and to then transport these discrete charge packets at thespeed of sound through the semiconductor material, as is described indetail in U.S. Pat. No. 4,893,161, entitled "Quantum-Well AcousticCharge Transport Device," issued to William J. Tanski, Sears W. Merritt,and Robert N. Sacks. The SAW thus functions similarly to the clockingsignal in a conventional charge-coupled device (CCD), but without theneed for the complex interconnections which CCD's require.

The very weak piezoelectricity of GaAs (k² =7.4×10⁻⁴ for the Rayleighmode on {100}-cut, <110>-propagating GaAs) dictates that the greatmajority of the energy in the SAW is manifested as mechanical energy andonly a small portion of the total energy is manifested through theelectrical potential which accompanies the SAW. It is this electricalcomponent of the total SAW energy which bunches the charge carriers toform distinct packets and which transports these packets, representingthe input signal, through the HACT device. Accordingly, present day ACTand HACT devices require large (about one Watt) acoustic power levels inorder to realize the voltage required (about one Volt) to effectcoherent charge packet transport within the HACT channel, synchronouswith the SAW clock signal.

Therefore, it is an object of the present invention to provide an HACTdevice which includes a greatly reduced acoustic power requirement forachieving coherent, synchronous charge transport.

SUMMARY OF THE INVENTION

In accomplishing the object of the present invention, a novel HACTdevice structure employing a thin-film overlay of another material isshown.

A heterostructure acoustic charge transport device includes asemiconductor substrate which has a surface and a source of electricalcharge. The semiconductor substrate also includes a surface acousticwave device which is coupled to the semiconductor substrate. The surfaceacoustic wave device operates in response to the applied electricalcharge source to transport electric charge.

The semiconductor substrate also includes a channel which is disposedalong the surface of the semiconductor substrate. The channel transportsthe electric charge in a particular direction in response to the surfaceacoustic wave device. Lastly, the semiconductor substrate includes apiezoelectric layer which is disposed over the channel. Thepiezoelectric layer facilitates transportation of the electric charge inthe particular direction.

The above and other objects, features, and advantages of the presentinvention will be better understood from the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a longitudinal cross-sectional view of an HACT deviceemploying a thin-film overlay of a dielectric and also another thin-filmlayer of material.

FIG. 2 is a graph of the electromechanical coupling coefficient versusZnO layer thickness on a semi-insulating {100}-cut, <110>-propagatingGaAs substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A HACT device employing a thin-film overlay of a more stronglypiezoelectric material can operate as a delay line and as a tapped delayline, or transversal filter, while requiring less total power for theSAW clock signal. The increased electrical potential per unit total SAWpower thus realized facilitates coupling between the total SAW energyand the mobile charge carriers.

Some materials systems, such as a GaAs substrate and a ZnO thin-filmoverlay, will require an intervening thin-film dielectric layer inbetween the HACT substrate and epitaxial layers and the thin-filmpiezoelectric overlay. This may be necessitated by chemical,semiconductor device processing, or adhesion incompatibilities betweenthe substrate material and the thin-film overlay material.

In acoustic charge transport (ACT), and in HACT technology, chargetransport occurs in piezoelectric semiconductors (for example,{100}-cut, <110>-propagating GaAs) when mobile charge is injected into,and trapped within the extrema of, the propagating electrical potentialwhich is associated with a SAW. Referring to FIG. 1, the semiconductorsubstrate 1 has incorporated upon it an interdigital metallic SAWtransducer pattern 2 for generating the SAW in response to an appliedexternal voltage at the appropriate frequency. The SAW transduction isenhanced by means of an appropriate thin-film overlay 3 of apiezoelectric material, such as zinc oxide (ZnO), as discussed in "GaAsMonolithic SAW Devices for Signal Processing and Frequency Control," byT. W. Grudkowski, G. K. Montress, M. Gilden, and J. F. Black, IEEECatalogue No. 80CH1602-2, pages 88-97, aluminum nitride (AlN), asdiscussed in "Growth and Properties of Piezoelectric and FerroelectricFilms," by M. H. Francome and S. V. Krishnaswamy, in Journal of VacuumScience and Technology A 8(3) , pages 1382-1390, or lithium niobate(LiNbO₃) or other ferroelectric material, as discussed in the above andin "Metalorganic Chemical Vapor Deposition of PbTiO₃ Thin Films," by B.S. Kwak, E. P. Boyd, and A. Erbil, in Applied Physics Letters, 53(18),pages 1702-1704. The travelling-wave electrical potential associatedwith the SAW is also enhanced by this overlay layer 3. The SAW thenpropagates from left to right, for example, encountering the edge of theepitaxial layers comprising the channel 4, sweeping past the inputcontact 5, where charge is injected into the minima of the SAWelectrical potential and entrained. The amount of the charge which isinjected into any particular minimum of the travelling-wave electricalpotential associated with the SAW is determined by the magnitude of theinput signal which is impressed upon the input contact 5 at the timethat the SAW electrical potential minimum passes by the input contact 5,and this particular charge packet magnitude remains invariant as thecharge packet thus formed moves through the HACT channel 4 in thatparticular electrical potential minimum at the speed of sound. The inputelectrode bias is formed by impressing a voltage difference between theinput ohmic contact 5 and the input Schottky gate 6. The charge packetsthen move beneath the nondestructive sensing (NDS) Schottky electrodearray 7, where the charge capacitively couples to the NDS electrodes 7and forms image charges which are a replica of the channel charge andhence of the charge representing the input signal. The image chargesinduced in these NDS electrodes 7 are combined to form a transversalfilter from which an output signal is derived. The charge packets arethen swept to the delay line output contact 9, where a delayed replicaof the input signal is extracted by application of a suitable biasvoltage applied between the delay line output Schottky gate contact 8and the delay line output contact 9. Finally, the SAW energy is incidentupon an acoustic absorber 10 which consists of a mass of a suitableviscous material, such as a room temperature vulcanizing silicone rubber(RTV), to avoid unwanted effects which can result from reflection of theSAW energy by the substrate 1 edges.

In the conventional HACT or ACT device, the very small (k² =7.4×10⁻⁴)SAW electromechanical coupling coefficient dictates that the bulk of theenergy in the SAW is available only as mechanical energy, rather than aselectrical potential. As such, large amounts of acoustic power must betransduced by the SAW transducer 2 in order to have available thevoltage level required in order to effect the bunching of chargecarriers, such as electrons, and the subsequent transport of the chargebunches, or packets, synchronously with the SAW, as is required formanifestation of the ACT phenomenon. The power level required in orderto achieve a given electrical potential is directly related to the totalacoustic power level present through the electromechanical couplingcoefficient, k².

As such, incorporation of a more strongly piezoelectric overlayer 3,providing greater electromechanical coupling between the SAW and theattendant electrical potential, results in reduced total acoustic powerrequirements to achieve a given electrical potential magnitude and soeffect the HACT phenomenon. The degree of change in acoustic powerrequirements which can be effected is illustrated by the graph given inFIG. 2, for the case of Rayleigh waves propagating in the <110>direction on a {100}-cut GaAs substrate 1, versus the thickness (inacoustic wavelengths) of a more strongly piezoelectric ZnO overlayer 3.As can be seen . .in.!. .Iadd.from .Iaddend.FIG. . .1.!..Iadd.2.Iaddend., very thin layers 3 of ZnO result in dramaticimprovements in the electromechanical coupling coefficient magnitude, toabout a thickness of 0.15 wavelengths. For thicker ZnO overlayers 3, ascan be seen in FIG. 1, the increase in electromechanical coupling isseen to saturate at a value of about k² =.Badd.0.01. In the more generalcase, the ratio of the acoustic powers required to achieve the sameelectrical potential in different substrates 1 and overlayers 3 is givenby:

    P.sub.a,x /P.sub.a,y =(k.sub.x.sup.2 C.sub.sx v.sub.ox)/(k.sub.y.sup.2 C.sub.s,y v.sub.oy)

where the acoustic power for the material X is denoted P_(a),x, with asimilar convention applying to the other substrate parameters beingcompared; k_(x) ² refers to the electromechanical coupling coefficient;C_(sx) refers to the characteristic capacitance per finger pair per cm.,and v_(ox) refers to the SAW velocity for that substrate 1 or compositematerials system.

For the particular case of a thin-film ZnO layer 3 in FIG. 1 on a GaAssubstrate 1, ranging from less than one to several micrometers inthickness, several different issues dictate the inclusion of a thin-filmdielectric layer 11, intervening between the ZnO layer 3 and the GaAssubstrate 1.

These issues devolve from the poor adhesion which ZnO exhibits on GaAssubstrates 1, evidenced by observed delamination of the thin-film layers3, the excellent adhesion which ZnO layers 3 exhibit on Si₃ N₄dielectric layers 11, the excellent adhesion of Si₃ N₄ layers 11 to GaAssubstrates 1, the electronic effects which zinc (a rapid diffusant and ap-type dopant) and oxygen (a deep trap, removing carriers from thematerial and re-emitting them at random times with very long timeconstants) exhibit when incorporated into the GaAs substrate 1 material,and the deleterious effects of ion bombardment of the GaAs substrate 1material during the sputtering process utilized for deposition andgrowth of ZnO films 3, which . .is.!. .Iadd.are .Iaddend.obviated by theinclusion of an intervening layer 11 of dielectric material.

The inclusion of a thin film layer of a more piezoelectric material 3 ona semiconductor substrate 1 which includes an HACT device thus resultsin substantial reduction of the acoustic energy level required in orderto manifest the HACT phenomenon. Further, some combinations of thin-filmoverlay 3 and substrate 1 materials will dictate the incorporation of athin-film dielectric layer 11.

When the thin-film piezoelectric layer 3 is also included over the SAWtransducer 2, a reduced area on the semiconductor substrate is required.This occurs through well-known relationships between SAW transducerparameters and results in greater SAW transducer 2 operating bandwidth.This increased bandwidth allows for greater variation in the SAWfrequency. This can be used to compensate for SAW transducer frequencyshifts introduced by temperature or manufacturing variations. Thereduced SAW transducer size allows for an increased number of HACTdevices per wafer and so for reduced cost per device.

Although the preferred embodiment of the invention has been illustrated,and that form described in detail, it will be readily apparent to thoseskilled in the art that various modifications may be made thereinwithout departing from the spirit of the invention or from the scope ofthe appended claims.

What is claimed is:
 1. A method for increasing the charge carryingcapacity of a heterostructure acoustic charge transport device,comprising the steps of:providing a semiconductor substrate having asurface; fabricating an interdigital metallic surface acoustic wavetransducer along said surface of said semiconductor substrate; providinga channel including epitaxial layers of semiconductor material alongsaid surface of said semiconductor substrate; .Iadd.and .Iaddend.depositing a . .thin-film.!. layer of piezoelectric material along saidchannel of said semiconductor substrate . .said thin-layer of dielectricmaterial being more strongly dielectric than said semiconductorsubstrate; providing electric charge to said channel of said substrate;and transporting said electric charge through said channel..!..Iadd.,wherein said layer of piezoelectric material is more stronglypiezoelectric than said semiconductor substrate. .Iaddend.
 2. A methodfor increasing the charge carrying capacity for a heterostructureacoustic charge transport device as claimed in claim 1, wherein there isfurther included the step of interposing a . .thin-film.!. dielectriclayer between said channel and said piezoelectric . .thin-film.!. layer,said . .thin-film.!. dielectric layer for promoting adhesion-of saidpiezoelectric . .thin-film.!. layer to said channel.
 3. A method forincreasing the charge carrying capacity of a heterostructure acousticcharge transport device as claimed in claim 1, wherein there is furtherincluded the step of attaching an acoustic absorber means . .attached.!.to said surface of said semiconductor substrate to prevent reflection ofsaid surface acoustic wave.
 4. A method for increasing the chargecarrying capacity of a heterostructure acoustic charge transport deviceas claimed in claim 1, wherein there is further included the stepsof:providing an input ohmic contact for coupling said electric charge tosaid channel; providing an input Schottky contact for controlling thetransportation of said electric charge through said channel, providing aSchottky nondestructive sensing electrode array coupled to said channelfor monitoring said transported electric charge; providing an outputSchottky contact for controlling the flow of electric charge throughsaid channel; and providing an output contact for receiving a replica ofsaid input electric charge.
 5. A method for reducing drive powerrequirements of a SAW transducer in a heterostructure acoustic chargetransport device, comprising the steps of:providing a semiconductorsubstrate having a surface; fabricating an interdigital metallic surfaceacoustic wave transducer along said surface of said semiconductorsubstrate; providing a channel including epitaxial layers ofsemiconductor material along said surface of said semiconductorsubstrate; .Iadd.and .Iaddend. depositing a . .thin-film.!. layer ofpiezoelectric material along said channel of said semiconductorsubstrate . .; providing electric charge to said channel of saidsubstrate; and transporting said electric charge through said channelwith approximately 50 milliWatts of power applied to said surface wavetransducer..!. .Iadd., wherein said layer of piezoelectric material ismore strongly piezoelectric than said semiconductor substrate. .Iaddend..Iadd.
 6. A method for increasing the charge carrying capacity of aheterostructure acoustic charge transport device, said method comprisingsteps of:providing a semiconductor substrate having a surface; disposingan interdigital metallic surface acoustic wave transducer on the surfaceof the semiconductor substrate; providing a channel including epitaxiallayers of semiconductor material along a <110> axis of the surface ofthe semiconductor substrate; and depositing a layer of piezoelectricmaterial along the channel of the semiconductor substrate, wherein thelayer of piezoelectric material is more strongly piezoelectric than thesemiconductor substrate. .Iaddend. .Iadd.7. A method as claimed in claim6, wherein said depositing step includes a step of depositing a layerconsisting of zinc oxide. .Iaddend. .Iadd.8. A method as claimed inclaim 7, wherein said step of depositing the layer of zinc oxideincludes a step of depositing a layer consisting of zinc oxide having athickness of about 0.15 acoustic wavelengths. .Iaddend. .Iadd.9. Amethod as claimed in claim 6, further including a step of interposing alayer comprising silicon nitride between the channel and the layer ofpiezoelectric material. .Iaddend. .Iadd.10. A method as claimed in claim6, wherein said step of providing the semiconductor substrate includes astep of providing a semiconductor substrate comprising gallium arsenideand wherein the surface is a {100}-oriented surface. .Iaddend. .Iadd.11.A method as claimed in claim 6, wherein said depositing step includes astep of depositing a layer of piezoelectric material wherein the layerof piezoelectric material is selected from a group consisting ofaluminum nitride, zinc oxide and lithium niobate. .Iaddend. .Iadd.12. Amethod as claimed in claim 6, wherein there is further included a stepof interposing a dielectric layer between the channel and the layer ofpiezoelectric material, the dielectric layer for promoting adhesion ofthe layer of piezoelectric material to the channel. .Iaddend. .Iadd.13.A method as claimed in claim 6, wherein said depositing step includes astep of depositing a layer of piezoelectric material consisting ofaluminum nitride. .Iaddend.